(1) Field of the Invention
The instant invention relates to fiber optic devices for distributed sensing applications, and more particularly to a sweep velocity-locked laser pulse generator (SV-LLPG) controlled using a digital phase locked loop (DPLL) circuit for the interrogation of sub-terahertz-range fiber structures and intrinsic Rayleigh backscatter along the length of an optical fiber.
(2) Description of Related Art
Sub-terahertz-range fiber structures (sub-THz-FS) have demonstrated their feasibility for distributed strain and temperature sensing applications. By definition, a sub-THz-FS is an optical fiber inline structure with characteristic geometries in the millimeter or sub-millimeter range that can be interrogated using sub-THz bandwidths in the optical frequency band. Uniquely, sub-THz-FSs allow systems to simultaneously achieve distributed strain and temperature measurements with high-accuracy and high spatial resolution using a narrow interrogation bandwidth. Previously, the interrogation system of sub-THz-FSs has been based on coherent optical frequency domain reflectometry (C-OFDR) techniques where the key component was a highly coherent swept laser source. More specifically, the laser source comprised an external cavity tunable laser (ECL). An ECL tunes its lasing wavelength via modulating the physical length of an externally-coupled cavity. Recent progress in micro-electro-mechanical systems (MEMS) technologies has led to miniaturized ECLs that can be as compact as conventional semiconductors. The unique advantage of ECLs include high coherence length and mode-hop-free broadband tuning (>100 nm or >12.5 THz at the 1.55 μm band). However, their disadvantages include inconsistent sweep velocity, non-repeatable starting wavelength, high system complexity with geometrically coupled moving optical components, and high cost. Several additional components are needed to compensate for these limitations. For example, an auxiliary sampling clock (k-clock) is employed to accommodate nonlinear sweep speeds and a wavelength reference gas cell is used to calibrate starting wavelength. Although effective, these methods add complexity, cost, and increased device footprint to ECLs.